Two bit non-volatile electrically erasable and programmable semiconductor memory cell utilizing asymmetrical charge trapping

ABSTRACT

A non-volatile electrically erasable programmable read only memory (EEPROM) capable of storing two bit of information having a nonconducting charge trapping dielectric, such as silicon nitride, sandwiched between two silicon dioxide layers acting as electrical insulators is disclosed. The invention includes a method of programming, reading and erasing the two bit EEPROM device. The nonconducting dielectric layer functions as an electrical charge trapping medium. A conducting gate layer is placed over the upper silicon dioxide layer. A left and a right bit are stored in physically different areas of the charge trapping layer, near left and right regions of the memory cell, respectively. Each bit of the memory device is programmed in the conventional manner, using hot electron programming, by applying programming voltages to the gate and to either the left or the right region while the other region is grounded. Hot electrons are accelerated sufficiently to be injected into the region of the trapping dielectric layer near where the programming voltages were applied to. The device, however, is read in the opposite direction from which it was written, meaning voltages are applied to the gate and to either the right or the left region while the other region is grounded. Two bits are able to be programmed and read due to a combination of relatively low gate voltages with reading in the reverse direction. This greatly reduces the potential across the trapped charge region. This permits much shorter programming times by amplifying the effect of the charge trapped in the localized trapping region associated with each of the bits. In addition, both bits of the memory cell can be individually erased by applying suitable erase voltages to the gate and either left or right regions so as to cause electrons to be removed from the corresponding charge trapping region of the nitride layer.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor memory devicesand more particularly to multi-bit flash electrically erasableprogrammable read only memory (EEPROM) cells that utilize the phenomenonof hot electron injection to trap charge within a trapping dielectricmaterial within the gate.

BACKGROUND OF THE INVENTION

Memory devices for non-volatile storage of information are currently inwidespread use today, being used in a myriad of applications. A fewexamples of non-volatile semiconductor memory include read only memory(ROM), programmable read only memory (PROM), erasable programmable readonly memory (EPROM), electrically erasable programmable read only memory(EEPROM) and flash EEPROM.

Semiconductor ROM devices, however, suffer from the disadvantage of notbeing electrically programmable memory devices. The programming of a ROMoccurs during one of the steps of manufacture using special maskscontaining the data to be stored. Thus, the entire contents of a ROMmust be determined before manufacture. In addition, because ROM devicesare programmed during manufacture, the time delay before the finishedproduct is available could be six weeks or more. The advantage, however,of using ROM for data storage is the low cost per device. However, thepenalty is the inability to change the data once the masks are committedto. If mistakes in the data programming are found they are typicallyvery costly to correct. Any inventory that exists having incorrect dataprogramming is instantly obsolete and probably cannot be used. Inaddition, extensive time delays are incurred because new masks mustfirst be generated from scratch and the entire manufacturing processrepeated. Also, the cost savings in the use of ROM memories only existif large quantities of the ROM are produced.

Moving to EPROM semiconductor devices eliminates the necessity of maskprogramming the data but the complexity of the process increasesdrastically. In addition, the die size is larger due to the addition ofprogramming circuitry and there are more processing and testing stepsinvolved in the manufacture of these types of memory devices. Anadvantage of EPROMs are that they are electrically programmed, but forerasing, EPROMs require exposure to ultraviolet (UV) light. Thesedevices are constructed with windows transparent to UV light to allowthe die to be exposed for erasing, which must be performed before thedevice can be programmed. A major drawback to these devices is that theylack the ability to be electrically erased. In many circuit designs itis desirable to have a non-volatile memory device that can be erased andreprogrammed in-circuit, without the need to remove the device forerasing and reprogramming.

Semiconductor EEPROM devices also involve more complex processing andtesting procedures than ROM, but have the advantage of electricalprogramming and erasing. Using EEPROM devices in circuitry permitsin-circuit erasing and reprogramming of the device, a feat not possiblewith conventional EPROM memory. Flash EEPROMs are similar to EEPROMs inthat memory cells can be programmed (i.e., written) and erasedelectrically but with the additional ability of erasing all memory cellsat once, hence the term flash EEPROM. The disadvantage of flash EEPROMis that it is very difficult and expensive to manufacture and produce.

The widespread use of EEPROM semiconductor memory has prompted muchresearch focusing on constructing better memory cells. Active areas ofresearch have focused on developing a memory cell that has improvedperformance characteristics such as shorter programming times, utilizinglower voltages for programming and reading, longer data retention times,shorter erase times and smaller physical dimensions. One such area ofresearch involves a memory cell that has an insulated gate. Thefollowing prior art reference is related to this area.

U.S. Pat. No. 4,173,766, issued to Hayes, teaches a metal nitride oxidesemiconductor (MNOS) constructed with an insulated gate having a bottomsilicon dioxide layer and a top nitride layer. A conductive gateelectrode, such as polycrystalline silicon or metal, is placed on top ofthe nitride layer. A major disadvantage of this device is the difficultyin using it to construct a flash EEPROM. A consequence of using anoxide-nitride structure as opposed to an oxide-nitride-oxide structureis that during programming the charge gets distributed across the entirenitride layer. The absence of the top oxide layer lowers the ability tocontrol where the charge is stored in the nitride layer.

Further, in the memory cell disclosed in Hayes, the nitride layer istypically 350 Angstroms thick. A thick nitride layer is required inHayes' device in order to achieve sufficient charge retention. Since thenitride can only tolerate relatively small internal electric fields, athick layer of nitride is required to compensate. Due to the thicknitride layer, very high vertical voltages are needed for erasing. Therelatively thick nitride layer causes the distribution of charge, i.e.,the charge trapping region, to be very wide and a wider charge trappingregion makes erasing the cell via the drain extremely difficult if notimpossible. In addition, drain erasing is made difficult because of theincreased thickness of the charge trapping layer. Thus, the memory celltaught by Hayes must have a thick nitride layer for charge retentionpurposes but at the expense of making it extremely difficult to erasethe device via the drain, thus making the device impractical for flashEEPROM applications.

To erase the memory cell of Hayes, the electrons previously trapped inthe nitride must be neutralized either by moving electrons out of thenitride or by transferring holes into the nitride. Hayes teaches anerase mode for his memory cell whereby the information stored on thenitride is erased by grounding the gate and applying a sufficientpotential to the drain to cause avalanche breakdown. Avalanche breakdowninvolves hot hole injection into the nitride in contrast to electroninjection. Avalanche breakdown, however, requires relatively highvoltages and high currents for the phenomenon to occur. To lower theavalanche breakdown voltage, a heavily doped impurity is implanted intothe channel between the source and the drain.

The hot holes are generated and caused to surmount the hole potentialbarrier of the bottom oxide and recombine with the electrons in thenitride. This mechanism, however, is very complex and it is difficult toconstruct memory devices that work in this manner. Another disadvantageof using hot hole injection for erase is that since the PN junctionbetween the drain and the channel is in breakdown, very large currentsare generated that are difficult to control. Further, the number ofprogram/erase cycles that the memory cell can sustain is limited becausethe breakdown damages the junction area. The damage is caused by thevery high local temperatures generated in the vicinity of the junctionwhen it is in breakdown.

In addition, it is impractical to use the memory device of Hayes in aflash memory array architecture. The huge currents generated duringerase using avalanche breakdown would cause significant voltage (i.e.,IR), drops along the bit line associated with the memory cell inbreakdown.

Another well known technique of erasing is to inject holes from the gateinto the nitride layer. This mechanism, however, is very complex anddifficult to control due to the higher mobility of holes versuselectrons in the nitride. With elevated temperatures, the highermobility of holes causes a large loss of charge retention andconsequently lower threshold voltage deltas from the originalprogramming threshold. Deep depletion phenomenon creates the need for acompanion serial device to control the programming/erase process.

U.S. Pat. No. 5,168,334, issued to Mitchell et al., teaches a singletransistor EEPROM memory cell. Mitchell, however, teaches anoxide-nitride-oxide (ONO) EEPROM memory cell wherein oxide-nitride-oxidelayers are formed above the channel area and between the bit lines forproviding isolation between overlying polysilicon word lines. Thenitride layer provides the charge retention mechanism for programmingthe memory cell.

Although the memory device of Mitchell includes a top oxide layer, it isnot very well suited for flash EEPROM applications. This is due to thevery wide charge trapping region that must be programmed in order toachieve a sufficient delta in the threshold voltage between programmingand reading. The Mitchell device is programmed and read in the forwarddirection. Since reading in the forward direction is less effective thanreading in the reverse direction, the charge trapping region must bewider by default in order to distinguish between the programmed andunprogrammed states. A wider charge trapping region, however, makes thememory device very difficult to erase, thus making this deviceinefficient for flash EEPROM applications.

A single transistor ONO EEPROM device is disclosed in the technicalarticle entitled “A True Single-Transistor Oxide-Nitride-Oxide EEPROMDevice,” T. Y. Chan, K. K. Young and Chenming Hu, IEEE Electron DeviceLetters, March 1987. The memory cell is programmed by hot electroninjection and the injected charges are stored in the oxide-nitride-oxide(ONO) layer of the device. This article teaches programming and readingin the forward direction. Thus, as in Mitchell, a wider charge trappingregion is required to achieve a sufficiently large difference inthreshold voltages between programming and reading. This, however, makesit much more difficult to erase the device.

Multi-bit transistors are known in the art. Most multi-bit transistorsutilize multi-level thresholds to store more than one bit with eachthreshold level representing a different state. A memory cell havingfour threshold levels can store two bits. This technique has beenimplemented in a ROM by using implanting techniques and has also beenattempted in FLASH and EEPROM memory. The multi-level thresholdtechnique has not been applied to EPROM memory due to the fact that ifthe window of a threshold defining a given state is exceeded, a UV erasecycle must be performed which is very cumbersome and costly. Inaddition, to perform the UV erase, the chip must first be removed fromthe system which can be very problematic.

Achieving multiple thresholds in FLASH and EEPROM requires an initialerase cycle to bring all the memory cells below a certain threshold.Then, utilizing a methodical programming scheme, the threshold of eachcell is increased until the desired threshold is reached. A disadvantagewith this technique is that the programming process requires constantfeedback which causes multi-level programming to be slow.

In addition, using this technique causes the window of operation todecrease meaning the margins for each state are reduced. This translatesto a lower probability of making good dies and a reduction in the levelof quality achieved. If it is not desired to sacrifice any margins whileincreasing the reliability of the cell, then the window of operationmust be increased by a factor of two. This means operating at muchhigher voltages which is not desirable because it lowers the reliabilityand increases the disturbances between the cells. Due to the complexityof the multi-threshold technique, it is used mainly in applicationswhere missing bits can be tolerated such as in audio applications.

Another problem with this technique is that the threshold windows foreach state may change over time reducing the reliability. It must beguaranteed that using the same word line or bit line to program othercells will not interfere with or disturb the data in cells alreadyprogrammed. In addition, the programming time itself increases toaccommodate the multitude of different programming thresholds. Thus, theshifting of threshold windows for each state over time reduces thewindow of operation and consequently increases the sensitivity todisturbs.

The reduced margins for the threshold windows for the multiple statesresults in reduced yield. Further, in order to maintain quality andthreshold margins, higher voltages are required. This implies higherelectric fields in the channel which contributes to lower reliability ofthe memory cell.

In order to construct a multi-bit ROM memory cell, the cell must havefour distinct levels that can be programmed. In the case of two levels,i.e., conventional single bit ROM cell, the threshold voltage programmedinto a cell for a ‘0’ bit only has to be greater than the maximum gatevoltage, thus making sure the cell does not conduct when it is turned onduring reading. It is sufficient that the cell conducts at least acertain amount of current to distinguish between the programmed andunprogrammed states. The current through a transistor can be describedby the following equation.$I = {\frac{1}{Leff}{K( {V_{G} - V_{T}} )}}$L_(eff) is the effective channel length, K is a constant, V_(G) is thegate voltage and V_(T) is the threshold voltage. However, in themulti-bit case, different thresholds must be clearly distinguishablewhich translates into sensing different read currents and slower readspeed. Further, for two bits, four current levels must be sensed, eachthreshold having a statistical distribution because the thresholdscannot be set perfectly. In addition, there will be a statisticaldistribution for the effective channel length which will further widenthe distribution of the read currents for each threshold level.

The gate voltage also affects the distribution of read currents. For thesame set of threshold levels, varying the gate voltage directly resultsin a variation of the ratio between the read currents. Therefore thegate voltage must be kept very stable. In addition, since there aremultiple levels of current, sensing becomes more complex than in the twolevel, i.e., single bit, cell.

The following prior art references are related to multi-bitsemiconductor memory cells.

U.S. Pat. No. 5,021,999, issued to Kohda et al., teaches a non-volatilememory cell using an MOS transistor having a floating gate with twoelectrically separated segmented floating gates. The memory cell canstore three levels of data: no electrons on either segment, electronsinjected into either one of the two segments and electrons injected intoboth segments.

U.S. Pat. No. 5,214,303, issued to Aoki, teaches a two bit transistorwhich comprises a semiconductor substrate, a gate electrode formed onthe substrate, a pair of source/drain regions provided in the substrateand an offset step portion formed in at least one of the source/drainregions and downwardly extending into the substrate in the vicinity ofthe gate electrode.

U.S. Pat. No. 5,394,355, issued to Uramoto et al., teaches a ROM memoryhaving a plurality of reference potential transmission lines. Eachreference potential transmission line represents a different level orstate. Each memory cell includes a memory cell transistor able toconnect one of the reference potential transmission lines to thecorresponding bit line.

U.S. Pat. No. 5,414,693, issued to Ma et al., teaches a two bit splitgate flash EEPROM memory cell structure that uses one select gatetransistor and two floating gate transistors. In this inventionessentially each bit is stored in a separate transistor.

U.S. Pat. No. 5,434,825, issued to Harari, teaches a multi-bit EPROM andEEPROM memory cell which is partitioned into three or more ranges ofprogramming charge. The cell's memory window is widened to store morethan two binary states. Each cell is programmed to have one of theprogrammed states. To achieve more than two binary states, multiplenegative and positive threshold voltages are used. The cell basicallycomprises a data storage transistor coupled to a series pass transistor.The data transistor is programmed to one of the predefined thresholdstates. Sensing circuitry distinguishes the different current levelsassociated with each programmed state.

SUMMARY OF THE INVENTION

The present invention discloses an apparatus for and method ofprogramming, reading and erasing a two bit flash electrically erasableprogrammable read only memory (EEPROM). The two bit flash EEPROM memorycell is constructed having a charge trapping non-conducting dielectriclayer sandwiched between two silicon dioxide layers. The nonconductingdielectric layer functions as an electrical charge trapping medium. Thecharge trapping layer is sandwiched between two layers of silicondioxide which act as electrical insulators. A conducting gate layer isplaced over the upper silicon dioxide layer. The two individual bits,i.e., left and right bits, are stored in physically different areas ofthe charge trapping region.

A novel aspect of the memory device is that while both bits areprogrammed in the conventional manner, using hot electron programming,each bit is read in a direction opposite that in which it was programmedwith a relatively low gate voltage. For example, the right bit isprogrammed conventionally by applying programming voltages to the gateand the drain while the source is grounded. Hot electrons areaccelerated sufficiently to be injected into a region of the trappingdielectric layer near the drain. The device, however, is read in theopposite direction from which it was written, meaning voltages areapplied to the gate and the source while the drain is grounded. The leftbit is similarly programmed and read by swapping the functionality ofsource and drain terminals. Programming one of the bits leaves the otherbit with its information intact and undisturbed. Programming one of thebits does, however, have a very small effect on the other bit, e.g.,slightly slower programming speed for the second bit.

Reading in the reverse direction is most effective when relatively lowgate voltages are used. A benefit of utilizing relatively low gatevoltages in combination with reading in the reverse direction is thatthe potential drop across the portion of the channel beneath the trappedcharge region is significantly reduced. A relatively small programmingregion or charge trapping region is possible due to the lower channelpotential drop under the charge trapping region. This permits muchfaster programming times because the effect of the charge trapped in thelocalized trapping region is amplified. Programming times are reducedwhile the delta in threshold voltage between the programmed versusunprogrammed states remains the same as when the device is read in theforward direction.

Another major benefit is that the erase mechanism of the memory cell isgreatly enhanced. Both bits of the memory cell can be erased by applyingsuitable erase voltages to the gate and the drain for the right bit andto the gate and the source for the left bit so as to cause electrons tobe removed from the charge trapping region of the nitride layer.Electrons move from the nitride through the bottom oxide layer to thedrain or the source for the right and the left bits, respectively.Another benefit includes reduced wearout from cycling thus increasingdevice longevity. An effect of reading in the reverse direction is thata much higher threshold voltage for the same amount of programming ispossible. Thus, to achieve a sufficient delta in the threshold voltagebetween the programmed and unprogrammed states of the memory cell, amuch smaller region of trapped charge is required when the cell is readin the reverse direction than when the cell is read in the forwarddirection.

The erase mechanism is enhanced when the charge trapping region is madeas narrow as possible. Programming in the forward direction and readingin the reverse direction permits limiting the width of the chargetrapping region to a narrow region near the drain (right bit) or thesource. This allows for much more efficient erasing of the memory cell.

Further, utilizing a thinner silicon nitride charge trapping layer thanthat disclosed in the prior art helps to confine the charge trappingregion to a laterally narrower region near the drain. Further, thethinner top and bottom oxide sandwiching the nitride layer helps inretention of the trapped charge.

In addition, unlike prior art floating gate flash EEPROM memory cells,the bottom and top oxide thickness can be scaled due to the deeptrapping levels that function to increase the potential barrier fordirect tunneling. Since the electron trapping levels are so deep,thinner bottom and top oxides can be used without compromising chargeretention.

Another benefit of localized charge trapping is that during erase, theregion of the nitride away from the drain does not experience deepdepletion since the erase occurs near the drain only. The finalthreshold of the cell after erasing is self limited by the devicestructure itself. This is in direct contrast to conventional singletransistor floating gate flash memory cells which are plagued with deepdepletion problems. To overcome these problems, manufacturers includecomplex circuitry to control the erase process in order to prevent orrecover from deep depletion.

Another approach previously employed in the prior art to solve the deepdepletion problem was to design the floating gate flash memory cellusing a split gate design forming multiple transistors per cell. Thesplit gate or double transistor constructions were necessary because theinformation carrying transistor, i.e., the floating gate transistor,potentially could be over erased. An over erase condition caused thethreshold voltage of the cell to go too low. The second transistor,acting as a control transistor, prevented the floating gate transistorfrom being over erased.

Considerations of the two bit memory cell of the present invention aredescribed hereinbelow. The first is the fact that reading in the reversedirection permits read through of the trapping region associated withthe other bit. The second is that programming the device to a low V_(T),by clamping the word line voltage V_(WL), further enhances the marginfor each bit. The margin is defined as the parameters that will programone of the bits without affecting the other.

Further, the locality of the trapped charge due to hot electroninjection in combination with reverse reading permits two distinctcharge trapping regions to be formed in a relatively short device whoseL_(eff) is approximately 0.2 microns. Also, utilizing a combination ofpositive V_(D) and either zero or negative V_(G) permits each bit to beerased separately.

The memory device also exhibits little or no disturb during programming.This is because during programming the drain voltage is only applied tothe junction adjacent to the region where charge trapping is to occur.

It is important to note that a memory cell constructed in accordancewith the present invention cannot store two bits utilizing programmingin the forward direction and reading in the forward direction. This isdue to the forward read requiring a wider charge trapping region to beprogrammed in order to achieve a sufficient delta in read currents for aone and a zero. Once one of the bits is programmed, the wider chargetrapping region prevents read through to the other bit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 illustrates a sectional view of a single bit flash EEPROM cell ofthe prior art utilizing Oxide-Nitride-Oxide (ONO) as the gatedielectric;

FIG. 2 illustrates a sectional view of a two bit flash EEPROM cellconstructed in accordance with an embodiment of the present inventionutilizing ONO as the gate dielectric;

FIG. 3 illustrates a sectional view of a two bit flash EEPROM cellconstructed in accordance with an embodiment of the present inventionutilizing a silicon rich silicon dioxide with buried polysilicon islandsas the gate dielectric;

FIG. 4 is a graph illustrating the threshold voltage as a function ofprogramming time for reading in the forward and reverse directions of aselected memory cell in accordance with this invention;

FIG. 5A illustrates a sectional view of a flash EEPROM cell of the priorart showing the area of charge trapping under the gate;

FIG. 5B illustrates a sectional view of a flash EEPROM cell constructedin accordance with an embodiment of the present invention showing thearea of charge trapping under the gate;

FIG. 6 is a graph illustrating the difference in threshold voltage inthe forward and reverse directions as a function of drain voltage for aflash EEPROM cell of the present invention that has been programmed;

FIG. 7 is a graph illustrating the difference in drain current in theforward and reverse directions as a function of drain voltage for aflash EEPROM cell of the present invention that has been programmed;

FIG. 8 is a graph illustrating the threshold voltage of a flash EEPROMcell of the present invention as a function of programming time forreading in the forward and reverse directions;

FIG. 9 is a graph illustrating the leakage current through the region oftrapped charge as a function of the voltage across the charge trappingregion while reading in the reverse direction;

FIG. 10 is a graph illustrating the gate voltage required to sustain agiven voltage in the channel beneath the edge of the region of trappedcharge while reading in the reverse direction;

FIG. 11 is a graph illustrating the effect of the gate voltage appliedduring reading on the difference in drain current between reading in theforward versus the reverse direction;

FIG. 12 is a graph illustrating the effect of the gate voltage (asmeasured by threshold channel current I_(TH)) on the difference inthreshold voltage between the forward read and reverse read directions;

FIG. 13 is a graph illustrating the effect programming one of the bitshas on the other bit that has not been previously programmed;

FIG. 14 is a graph illustrating the effect programming one of the bitshas on the other bit that has been previously programmed;

FIG. 15 is a sectional view of a two bit EEPROM cell constructed inaccordance with an embodiment of the present invention showing the areaof charge trapping under the gate for both the right and the left bits;

FIG. 16 is a graph illustrating the effect of a low drain voltage on theread through of a programmed bit;

FIG. 17 is a graph illustrating the effect of programming on erase forthe forward and reverse directions;

FIG. 18 is a graph illustrating the separate bit erase capability of thetwo bit EEPROM memory cell of the present invention;

FIG. 19 is a graph illustrating the effect of cycling on the program anderase ability of the two bit EEPROM cell of the present invention;

FIG. 20 is a graph illustrating the effect of over programming on theability to erase for the forward and reverse directions;

FIG. 21 is a graph illustrating the programming and erasing curves forusing oxide versus TEOS as the material used as the top oxide;

FIG. 22 is a graph illustrating the erase curves for two differentvalues of drain voltage while the gate is held at ground potential;

FIG. 23 is a graph illustrating the erase curve for two different valuesof gate voltage;

FIG. 24A illustrates a sectional view of a flash EEPROM cell of theprior art showing the area of charge trapping under the gate after beingprogrammed for a period of time; and

FIG. 24B illustrates a sectional view of a flash EEPROM cell constructedin accordance with an embodiment of the present invention showing thearea of charge trapping under the gate after being programmed for asufficient time to achieve the same threshold voltage of the cellillustrated in FIG. 24A.

DETAILED DESCRIPTION OF THE INVENTION

The two bit flash EEPROM cell of the present invention can best beunderstood with an understanding of how present day single bit chargetrapping dielectric flash EEPROM memory cells are constructed,programmed and read. Thus, prior art single bit ONO EEPROM memory cellsand the conventional method used to program, read and erase them aredescribed in some detail. Illustrated in FIG. 1 is a cross section of aconventional ONO EEPROM memory cell as disclosed in the technicalarticle entitled “A True Single-Transistor Oxide-Nitride-Oxide EEPROMDevice,” T. Y. Chan, K. K. Young and Chenming Hu, IEEE Electron DeviceLetters, March 1987, incorporated herein by reference. The memory cell,generally referenced 41, comprises a P-type silicon substrate 30, two PNjunctions between N+ source and drain regions 32, 34 and P typesubstrate 30, a non conducting nitride layer 38 sandwiched between twooxide layers 36, 40 and a polycrystalline conducting layer 42.

Programming Prior Art Single Bit Memory Devices

The operation of the prior art memory cell 41 will now be described. Toprogram or write the cell, voltages are applied to the drain 34 and thegate 42 and the source 32 is grounded. For example, 10V is applied tothe gate and 9V is applied to the drain. These voltages generate avertical and lateral electric field along the length of the channel fromthe source to the drain. This electric field causes electrons to bedrawn off the source and begin accelerating towards the drain. As theymove along the length of the channel, they gain energy. If they gainenough energy they are able to jump over the potential barrier of theoxide layer 36 into the silicon nitride layer 38 and become trapped. Theprobability of this occurring is a maximum in the region of the gatenext to the drain 34 because it is near the drain that the electronsgain the most energy. These accelerated electrons are termed hotelectrons and once injected into the nitride layer they become trappedand remain stored there. The trapped electrons cannot spread through thenitride layer because of the low conductivity of the nitride layer andthe low lateral electric field in the nitride. Thus, the trapped chargeremains in a localized trapping region in the nitride typically locatedclose to the drain.

In U.S. Pat. No. 4,173,766, issued to Hayes, the nitride layer isdescribed as typically being about 350 Angstroms thick (see column 6,lines 59 to 61). Further, the nitride layer in Hayes has no top oxidelayer. A top oxide layer would serve as a low conductivity layer toprevent holes from moving into the nitride from the overlying gate andcombining with electrons trapped in the nitride which reduces the chargestored in the nitride. If the memory cell of Hayes used a thinnernitride layer, then electrons trapped in the nitride layer would be lostto holes entering from the overlying conductive gate. The conductivegate permits the electrons in the nitride to be removed. Further, oncethe electrons are trapped in a given region of the nitride associatedwith a single cell, the programming of adjacent cells can cause anelectric field to be generated with respect to the electrons in thetrapped region of the single cell causing further dissipation of theelectrons from the trapped region. During life testing, where the deviceis subjected to elevated temperatures typically in the range from about150 degrees Centigrade to 250 degrees Centigrade, holes from the gatecan enter the nitride and combine with the electrons to further reducethe amount of charge trapped in the nitride. Although lateral fieldsexist in the nitride of the Hayes structure as they do in any ONOstructure used as gate insulation in an MOS device, the relatively thicknitride layer such as disclosed by Hayes causes the electrons to movelaterally in response to this lateral field and come to rest either intraps between the conduction and valence bands or in localized regionsof positive charge in the nitride layer. Such movement of electrons,commonly known as electron hopping, can readily occur in a relativelythick nitride layer such as disclosed by Hayes. Such hopping diffusesand thus reduces the localized intensity of the trapped charge.

As previously described, in order to achieve an effective delta inthreshold voltage between the unprogrammed and the programmed state ofeach cell, the charge trapping region of prior art flash EEPROM cellsmust be made fairly wide. Thus, electrons are trapped in areas far fromthe drain which directly affects the effectiveness of the erase. In somecases, the device cannot be erased at all because the charge trappingregion was programmed too wide.

In memory cells constructed using a conductive floating gate, the chargethat gets injected into the gate is distributed equally across theentire gate. The threshold voltage of the entire gate increases as moreand more charge is injected into the gate. The threshold voltageincreases because the electrons that become stored in the gate screenthe gate voltage from the channel.

With reference to FIG. 1, in devices with low conductivity or nonconductive floating gates, the injection of hot electrons into thesilicon nitride layer causes the gate threshold voltage to increase onlyin the localized trapping region. This is in contrast to the conductivefloating gate memory cells of EPROMs and EEPROMs wherein the gatethreshold voltage of the entire channel rises as programming timeincreases. In both conductive and non conductive floating gate memorycell designs, an increase in the gate threshold voltage causes thecurrent flowing through the channel to decrease for a given gatevoltage. This reduces programming efficiency by lengthening theprogramming time. However, due to the localized electron trapping in thenon conductive floating gate memory cell design, the programming time isreduced less than with the conductive floating gate memory cell design.The technique of programming flash EEPROM memory cells with eitherconductive or low conductivity or non conductive floating gates is wellknown in the art and is currently used to program EEPROM and flashEEPROM memory cells.

Reading Prior Art Single Bit Memory Devices

The method of reading prior art flash EEPROM memory cells will now bedescribed. The conventional technique of reading both prior artconductive floating gate and non conductive localized trapping gateEEPROM or flash EEPROM memory is to apply read voltages to the gate anddrain and to ground the source. This is similar to the method ofprogramming with the difference being that lower level voltages areapplied during reading than during programming. Since the floating gateis conductive, the trapped charge is distributed evenly throughout theentire floating conductor. In a programmed device, the threshold istherefore high for the entire channel and the process of reading becomessymmetrical. It makes no difference whether voltage is applied to thedrain and the source is grounded or vice versa. A similar process isalso used to read prior art non conductive localized gate flash EEPROMdevices.

The process of programming typically includes writing followed byreading. This is true for all EPROM and EEPROM memory devices. A shortprogramming pulse is applied to the device followed by a read. The readis actually used to effectively measure the gate threshold voltage. Byconvention, the gate threshold voltage is measured by applying a voltageto the drain and a separate voltage to the gate, with the voltage on thegate being increased from zero while the channel current flowing fromdrain to source is measured. The gate voltage that provides 1 p.A ofchannel current is termed the threshold voltage.

Typically, programming pulses (i.e., write pulses) are followed by readcycles wherein the read is performed in the same direction that theprogramming pulse is applied. This is termed symmetrical programming andreading. Programming stops when the gate threshold voltage has reached acertain predetermined point (i.e., the channel current is reduced to asufficiently low level). This point is chosen to ensure that a ‘0’ bitcan be distinguished from a ‘1’ bit and that a certain data retentiontime has been achieved.

The Two Bit Memory Device of the Present Invention

A sectional view of a two bit flash EEPROM cell constructed inaccordance with an embodiment of the present invention utilizing ONO asthe gate dielectric is shown in FIG. 2. The flash EEPROM memory cell,generally referenced 10, comprises a P-type substrate 12 having twoburied PN junctions, one being between the source 14 and substrate 12,termed the left junction and the other being between the drain 16 andthe substrate 12, termed the right junction. Above the channel is alayer of silicon dioxide 18, preferably between approximately 60 to 100Angstroms thick, which forms an electrical isolation layer over thechannel. On top of the silicon dioxide layer 18 is a charge trappinglayer 20 constructed preferably in the range of 20 to 100 Angstromsthick and preferably comprised of silicon nitride, Si₃N₄. The hotelectrons are trapped as they are injected into the charge trappinglayer. In this fashion, the charge trapping layer serves as the memoryretention layer. Note that the programming, reading and erasing of thememory cell of the present invention is based on the movement ofelectrons as opposed to movement of holes. The charge trappingdielectric can be constructed using silicon nitride, silicon dioxidewith buried polysilicon islands or implanted oxide, for example. In thethird listed alternative, the oxide can be implanted with arsenic, forexample. The thickness of layer 18 is chosen to be in excess of 50Angstroms to prevent electrons from tunneling through the oxide andleaving charge trapping layer 20 during the operation of the cell. Thusthe lifetime of the cell of this invention is greatly extended relativeto prior art MNOS devices. The memory cell 10 is capable of storing twobits of data, a right bit represented by the dashed circle 23 and a leftbit represented by the dashed circle 21.

It is important to note that the two bit memory cell of the presentinvention is a symmetrical device. Therefore, the terms source and drainas used with conventional one bit devices may be confusing. In reality,the left junction serves as the source terminal and the right junctionserves as the drain terminal for the right bit. Similarly, for the leftbit, the right junction serves as the source terminal and the leftjunction serves as the drain terminal. Thus, to avoid confusion, theterms left or first junction and right or second junction are utilizedmost of the time rather than source and drain. When the distinctionbetween left and right bits is not crucial to the particular discussion,the terms source and drain are utilized. However, it should beunderstood that the source and drain terminals for the second bit arereversed compared to the source and drain terminals for the first bit.

Another layer of silicon dioxide 22 is formed over the charge trappinglayer, (i.e., silicon nitride layer), and is preferably betweenapproximately 60 to 100 Angstroms thick. The silicon dioxide layer 22functions to electrically isolate a conductive gate 24 formed over thesilicon dioxide layer 22 from charge trapping layer 20. The thickness ofgate 24 is approximately 4,000 Angstroms. Gate 24 can be constructedfrom polycrystalline silicon, commonly known as polysilicon.

Charge trapping dielectric materials other than nitride may also besuitable for use as the asymmetric charge trapping medium. One suchmaterial is silicon dioxide with buried polysilicon islands. The silicondioxide with polysilicon islands is sandwiched between two layers ofoxide in similar fashion to the construction of the ONO memory cell inFIG. 2. A sectional view of a two bit flash EEPROM cell constructed inaccordance with a preferred embodiment of the present inventionutilizing a silicon rich silicon dioxide layer 54 with buriedpolysilicon islands 57 as the gate dielectric is illustrated in FIG. 3.Note that for simplicity, only a few polysilicon islands are numbered. AP-type substrate 62 has buried N+ source 58 and N+ drain 60 regions. Thesilicon dioxide 54 with buried polysilicon islands 57 is sandwichedbetween two layers of silicon dioxide 52, 56. Covering oxide layer 52 ispolysilicon gate 50. Gate 50 is typically heavily doped with an N-typeimpurity such as phosphorus in the 10¹⁹ to 10²⁰ atom/cc range. Similarto the two bit memory cell of FIG. 2, the memory cell of FIG. 3 iscapable of storing two data bits, a right bit represented by the dashedcircle 55 and a left bit represented by the dashed circle 53. Theoperation of the memory cell of FIG. 3 is similar to that of the memorycell illustrated in FIG. 2 with programming and reading occurring inopposite directions for each bit.

Alternatively, the charge trapping dielectric can be constructed byimplanting an impurity, such as arsenic, into a middle layer 54 ofsilicon dioxide deposited on top of the bottom oxide 56.

A key aspect of the present invention lies in the manner in which theflash EEPROM memory cell 10 (FIG. 2) is programmed and read. Rather thanperforming symmetrical programming and reading, the flash EEPROM memorycell of the present invention is programmed and read asymmetrically.This means that programming and reading occur in opposite directions.The arrows labeled PROGRAM and READ for each bit (i.e. the left bit andthe right bit) in FIG. 2 point in opposite directions to signify thisasymmetry. Thus, programming is performed in what is termed the forwarddirection and reading is performed in what is termed the opposite orreverse direction.

It is noted that throughout the discussion of the EEPROM memory cell ofthe present invention presented below, the voltage levels discussed inconnection therewith are assumed to be independent of the power supplyvoltage. Thus, the power supply voltages supplied to the chip embodyingthe EEPROM memory device may vary while the voltages applied to thegate, drain and source thereof will be supplied from regulated voltagesources.

Programming One Bit in the Forward Direction

As previously mentioned, the flash EEPROM memory cell 10 of FIG. 2 isprogrammed similarly to the prior art flash EEPROM memory cell ofFIG. 1. Voltages are applied to the gate 24 and drain 16 creatingvertical and lateral electrical fields which accelerate electrons fromthe source 14 along the length of the channel. As the electrons movealong the channel some of them gain sufficient energy to jump over thepotential barrier of the bottom silicon dioxide layer 18 and becometrapped in the silicon nitride layer 20. For the right bit, for example,the electron trapping occurs in a region near the drain 16 indicated bythe dashed circle 23 in FIG. 2. Thus the trapped charge is self-alignedto the junction between the drain 16 and the substrate. Electrons aretrapped in the portion of nitride layer 20 near but above andself-aligned with the drain region 16 because the electric fields arethe strongest there. Thus, the electrons have a maximum probability ofbeing sufficiently energized to jump the potential barrier of thesilicon dioxide layer 18 and become trapped in the nitride layer 20 nearthe drain 16. The threshold voltage of the portion of the channelbetween the source 14 and drain 16 under the region of trapped chargeincreases as more electrons are injected into the nitride layer 20.

It is important to note that in order to be able to subsequently erasememory device 10 effectively, the programming time period must belimited. As the device continues to be programmed, the width of thecharge trapping region increases. If programming continues past acertain point the charge trapping region becomes too wide wherebyerasing is ineffective in removing trapped charge from the nitride layer20.

However, by reading in the reverse direction, programming times can beshortened. This permits a much narrower charge trapping region. This inturn greatly increases the erase efficiency since fewer electrons needto be removed to erase the device. In addition, the trapped electronsare stored in a narrower region near the drain also improving theeffectiveness of the erase.

Reading One Bit in the Forward Direction

If the flash EEPROM memory cell 10 is read using the conventionaltechnique of reading in the same direction as programming, the timeneeded to program the device greatly increases to achieve the samethreshold voltage. Reading in the same direction as programming meansthe device is programmed and read in the same forward direction. Duringreading, voltages having levels lower than the voltages applied duringprogramming are applied to the gate and drain and the channel currentare sensed. If device 10 is programmed (i.e., a logic ‘0’) the channelcurrent should be very low and if the device is not programmed (i.e., alogic ‘1’) there should be significant channel current generated.Preferably, the difference in the channel current between the ‘0’ and‘1’ logic states should be maximized in order to better distinguishbetween the ‘0’ and ‘1’ logic states.

Illustrated in FIG. 4 is a graph showing the rise in gate thresholdvoltage as a function of programming time for reading in the forwarddirection (curve labeled FORWARD READ) and for reading in the reversedirection (curve labeled REVERSE READ). Apparent from the graph in FIG.4 is the several orders of magnitude reduction in programming timeachieved when reading in the reverse direction versus reading in theforward direction. As is described in more detail below, this dramaticreduction in programming time is due to amplification of the effect ofthe trapped charge injected into the nitride layer brought about byreading the memory cell in the opposite direction from which it wasprogrammed.

As stated above, the time needed to program the flash EEPROM memory cellgreatly increases when reading occurs in the same direction (i.e., theforward direction) as programming. The reason for this will now beexplained in more detail with reference to FIGS. 5A and 5B. FIG. 5Aillustrates a sectional view of a flash EEPROM cell of the prior artshowing the area 66 of charge trapping under the gate 42. FIG. 5Billustrates a sectional view of a flash EEPROM cell constructed inaccordance with an embodiment of the present invention showing the area68 of charge trapping under the gate 24 for the right bit.

A description of what occurs during programming is presented firstfollowed by what occurs during reading. Note that the description thatfollows also pertains to the memory cell of FIG. 3 comprising thesilicon dioxide layer 54 having buried polysilicon islands 57substituting for the nitride layer 20 of FIG. 2. During programming, hotelectrons are injected into the nitride layer 20, as described above.Since the nitride 20 is a nonconductor, the trapped charge remainslocalized to the region near the drain 34 (FIG. 5A) or 16 (FIG. 5B). Theregion of trapped charge is indicated by the cross hatched area 66 inFIG. 5A and by the cross hatched area 68 in FIG. 5B. Thus, the thresholdvoltage rises, for example, to approximately 4 V, only in the portion ofthe channel under the trapped charge. The threshold voltage of theremainder of the channel under the gate remains at, for example,approximately 1 V. If the device is now read in the conventional forwarddirection (i.e., voltages are applied to the gate and drain as indicatedby the arrow in FIG. 5A), electrons move off the source and begintraveling toward the drain. When a logic ‘0’ is programmed, there can belittle or no channel current through the device when it is read. Thus,only if a sufficient portion of the channel is turned off, can theelectron current be stopped. If the channel cannot be completely turnedoff, the electrons will reach the drain. Whether the electrons reach thedrain will be determined by, among other things, the length of thetrapping area. If the memory cell is programmed for a sufficiently longperiod, eventually, the channel stops conducting when read in theforward direction. If the trapped charge region (the programmed area) 66(FIG. 5A) is not long enough, electrons can punch through to the drain34 in the depletion region under the trapped charge 66.

When the device is read in the forward direction, a voltage is appliedto the drain and the gate, for example 2V and 3V, respectively, and thesource is grounded. Full inversion occurs in the channel under the areaof the nitride 38 that does not have trapped charge. A vertical electricfield exists in the channel that spans the length of the channel up tothe region of the channel underneath the trapped charge 66. In theinversion region, electrons travel in a linear fashion up to the edge 35of the inversion region which is beneath the left edge 35 of the trappedcharge region 66. This is indicated by the line shown in the channelregion in FIG. 5A that extends from the source to just beneath the edge33 of the region of trapped charge 66. Due to the fact that the deviceis in inversion (i.e., the channel is in a conductive state), thepotential in the inversion layer is pinned to ground potential becausethe source is grounded. The voltage in the inverted channel near thetrapped charge (i.e., just to the left of the right edge 35 of thechannel inversion region) is approximately zero. Thus, the voltageacross the region of trapped charge is close to the full drain potentialof 2 V. Due to the drain potential across the channel region beneath thetrapped charge 66 some of the electrons punch through across the trappedregion to the drain, resulting in a channel current.

The diagonal line under the channel in FIGS. 2 and 5A indicate thereduction in the number of electrons in the channel as a function ofchannel distance. The channel region under the trapped charge is off(i.e., not inverted) due to the high threshold voltage required toinvert this region under the trapped charge. However, the channel regioninside the dashed circle 23 in FIG. 2 and under the region 66 in FIG. 5Ais a depletion region because the device is in saturation (a device willbe in saturation when V_(DS), the voltage from drain to source, ishigher than V_(DSAT), the saturation voltage). Due to the voltage on thedrain 34, a lateral electric field exists in this portion of the channelunder region 66. As a result of this lateral electric field, anyelectron arriving at the edge of the depletion region will be sweptthrough and pulled to the drain 34. As described earlier, thisphenomenon is called punch through. Punch through occurs if the lateralelectric field is strong enough to draw electrons through to the drain,regardless of the threshold level. In order to prevent punch throughfrom occurring during a read, the prior art memory cells require a muchlonger programming time than does the memory cell of this inventionbecause the prior art memory cells are read in the forward direction. Asthe memory device is programmed for a longer and longer time, more andmore electrons are injected into the nitride, increasing the length ofthe programmed portion 66 (FIG. 5A) of the channel. The memory cell mustbe programmed for an amount of time that yields a trapped charge region66 of sufficient length to eliminate the punch through of electrons.When this occurs, the lateral electric field is too weak for electronsto punch through to the drain under normal operating conditions. As anexample, for the threshold voltage equaling 3V during read in theforward direction, FIG. 4 shows that at programming time ofapproximately 3 milliseconds is required.

Reading in the Reverse Direction

However, if the flash EEPROM memory cell 10 (FIG. 5B) is read in thereverse direction, a very different scenario exists. Reading in thereverse direction means reading in a direction opposite to that ofprogramming. In other words, voltages are applied to the source 14 andthe gate 24 and the drain 16 is grounded. Similar to the prior artmemory device of FIG. 5A, the memory device of FIG. 5B is programmed inthe forward direction by injecting hot electrons into region 68 of thenitride layer 20. Since nitride 20 is a nonconductor, the trapped chargeremains localized to the region near the drain, for the right bit, forexample. The left bit is similar except that source and drainfunctionality are reversed. The region of trapped charge is indicated bythe cross hatched area 68 in FIG. 5B. Thus, the threshold voltage rises,for example, to approximately 4V only in the portion of the channelunder the trapped charge 68. The threshold voltage of the remainder ofthe channel remains at, for example, approximately 1 V.

To read the right bit of the device of FIG. 5B in the reverse direction,a voltage is applied to the source 14 and the gate 24, for example 2Vand 3V, respectively, and the drain 16 is grounded. A major differencebetween reading in the forward direction and reading in the reversedirection is that when reading in the reverse direction, the gatevoltage required to put the channel of the memory device into inversionincreases significantly. For the same applied gate voltage of 3V, forexample, there will be no inversion but rather the channel of the memorydevice will be in depletion. The reason for this is that the channelregion next to the drain 16 (which functions as the source in read) isnot inverted due to the electron charge in region 68 of the nitride 20.The channel adjacent the source 14 (which functions as the drain inread) is not inverted because 2V is applied to the source 14 and thechannel, to be inverted, must be inverted relative to 2 V. In the caseof reading in the reverse direction, in order to sustain a highervoltage in the channel, a much wider depletion region must be sustained.A wider depletion region translates to more fixed charge that must becompensated for before there can be inversion. When reading in thereverse direction in accordance with the present invention, to achieve avoltage drop across the charge trapping region 66 of the prior artmemory device shown in FIG. 5A similar to the voltage drop achieved whenreading the same device in the forward direction, a higher gate voltageis required, for example, 4 V. This is in contrast to the prior artmemory device where the source was grounded and a lower gate voltage wasrequired to invert the channel. In the memory device of the presentinvention, a much higher gate voltage is required to pin the voltage inthe channel to a higher voltage, i.e., the 2V that is applied to thesource terminal rather than ground. In other words, the presentinvention recognizes and takes advantage of the fact that for the samemagnitude potential across the drain and the source, the voltage acrossthe portion of the channel under the trapped charge region 68 (FIG. 5B)is significantly reduced when reading occurs in a reverse direction towriting (programming) directly resulting in less punch through andgreater impact of the programming charge injected in region 68 of thenitride layer 20 (FIG. 5B) on the threshold voltage of the transistor.As an example, for the threshold voltage V_(T) equaling 3v duringreverse read, FIG. 4 shows that a programming time of approximately 2microseconds is required. This programming time is three orders ofmagnitude less than the programming time required for the same thresholdvoltage when the cell is read in the forward direction.

In the prior art, memory cells utilizing the ONO structure have haddifficulty retaining the localized charge in the nitride layer. This isbecause such memory cells are programmed in a first forward directionand then read in the same direction. The reading of the programmed cellin the forward direction requires a significant amount of charge to bestored on the nitride to provide the desired increase in thresholdvoltage associated with the programmed cell. However, in accordance withthis invention, by reading in the reverse direction, significantly lesscharge is required to be stored on the nitride to achieve the sameincrease in threshold voltage in a programmed cell. FIG. 4 shows thedifference in charge (measured as a function of programming timerequired to achieve a given threshold voltage V_(T)) for reading in thereverse direction versus the forward direction. In the prior art, thecharge retention in a localized region of the silicon nitride layer wasdifficult if not impossible to achieve because the lateral electricfield generated by the charge dispersed the charge laterally in thenitride layer. Such dispersion particularly occurred during the hightemperature retention bake required for quality control and reliability.The high temperature retention bake typically requires temperaturesbetween 150 degrees Centigrade to 250 degrees Centigrade for at least 12to 24 hours. The charge in the prior art devices typically dispersedthrough the nitride during the high temperature bake causing theperformance of prior art devices using the nitride layer as a chargeretention material to be less than satisfactory. Accordingly, prior artdevices that use the nitride layer for charge retention are not widelyused. In addition, charge stored on the nitride layer in prior artmemory cells is particularly prone to lateral diffusion and dispersionthrough the nitride layer in response to the retention bake due to theinternal fields causing what is known as electron hopping. Thephenomenon of electron hopping is exponentially dependent on the fieldstrength. In the case of charge in the nitride layer the internallygenerated electric field is directly related to the amount of chargestored on the nitride layer. Because electron hopping is exponentiallydependent upon the electric field strength, the additional chargerequired to obtain a given threshold voltage change or shift when thememory cell is read in the same direction as it was programmed causes avery significant change in the charge distribution in the nitride layer.This change in the charge distribution seriously degrades the thresholdvoltage from the intended (i.e., design) threshold voltage.Consequently, prior art ONO devices have not been successful.

In accordance with the present invention, by reading the memory cell inthe reverse direction from which the memory cell is programmed, theamount of charge required to achieve a given threshold voltage isreduced in some cases by a factor of two or three times the amount ofcharge required to obtain the same threshold voltage shift when thememory cell is read in the forward direction. Accordingly, the internalelectric fields generated by the charge in the nitride when the memorycell is to be read in the reverse direction are much less than theinternal electric fields associated with the charge stored on thenitride when the memory cell is to be read in the forward direction.Consequently electron hopping is exponentially reduced and the smallamount of charge stored in the nitride does not disperse laterallythrough the nitride due to the internally self generated electric fieldseven during retention bake. Consequently, the memory cell of the presentinvention does not suffer the degradation in performance and reliabilityof prior art ONO memory cells which are programmed and read in the samedirection.

Sample Flash EEPROM Device Data

Data obtained from flash EEPROM devices constructed in accordance withthe present invention will now be presented to help illustrate theprinciples of operation thereof. A graph illustrating the difference inthreshold voltage in the forward and reverse directions as a function ofdrain voltage for a flash EEPROM cell of the present invention that hasbeen previously programmed is shown in FIG. 6. The memory cell used toobtain the data presented in FIGS. 6, 7 and 8 was constructed with abottom oxide layer 18, a top oxide 22 and a nitride layer 20, each 100Angstroms thick. The drawn width of the channel measures 0.6 microns andthe drawn length of the channel measures 0.65 microns.

While reading in the forward direction, the threshold voltage isapproximately the same as the threshold voltage when reading in thereverse direction for low drain voltages. At low drain voltages there isinsufficient potential for punch through to occur. However, as the drainvoltage increases while reading in the forward direction, the punchthrough region increases resulting in lower threshold voltage. At a highenough drain voltage, the entire portion of the channel under thetrapped charge in region 68 of nitride layer 20 (FIG. 5B) is punchedthrough and the threshold voltage levels off at the original thresholdvoltage of the channel.

However, while reading in the reverse direction, the V_(T) versus V_(D))curve appears to follow the V, versus V_(D) curve while reading in theforward direction at low drain voltages. However, the curves rapidlydiverge for higher drain voltages and the threshold voltage for readingin the reverse direction levels off at approximately 4V. At a gatevoltage V_(G) of approximately 4V and a drain voltage V_(D) of 1.2V, thedevice has reached saturation (V_(DSAT)). At this gate voltage, anyfurther increase in V_(D) cannot be transferred through the inversionlayer thus establishing the maximum potential drop across the portion ofthe channel beneath the charge trapping region 68. The V_(T) thenbecomes independent of further increases in V_(D). For example, at adrain voltage of 1.6V, the difference in V_(T) between reverse andforward read is almost 2V.

A graph illustrating the difference in drain current in the forward andreverse directions as a function of drain voltage for a flash EEPROMcell of the present invention that has been programmed is shown in FIG.7. In FIG. 7, rather than measure threshold voltage, the drain currentis measured while keeping the gate voltage constant. In the forwarddirection, as expected, the drain current I_(D) increases as the drainvoltage V_(D) increases. The curve labeled FORWARD also resembles theI_(D) curve for reading an unprogrammed cell in the reverse direction.

The drain current while reading in the reverse direction also increaseswith increasing drain voltage (measured at the source which functions asthe drain when reading in the reverse direction) but the drain currentlevels off at a much lower current than when reading in the forwarddirection. The difference between drain currents at a V_(D) of 2 V is onthe order of approximately 1000 times. If the logic threshold for thismemory cell is set to 10 μA, the forward curve can represent a logic ‘0’and the reverse curve a logic ‘1’.

The Voltage V_(X) in the Channel

The voltage V_(X) is defined as the voltage in the channel at a distanceX from the source. Using the example presented above, the voltage V_(X)that exists in the channel of the memory cell of the present invention(FIG. 5B, for example) will not be 2V because the device is in depletionrather than inversion. On the other hand, the voltage V_(X) must belarger than 0 because a gate voltage of only 1.5V is able to sustainapproximately 0.4V in the channel. The actual voltage in the channelvaries across the channel length because of the lateral electric fieldset up between the source and the drain. The threshold voltage, however,varies as a function of the voltage in the channel.

With reference to FIG. 5B, the channel will be in saturation as long asthe gate voltage V_(G) is higher than the threshold voltage V_(T) andthe voltage V_(X) at any point in the channel is given byV_(X)=V_(DSAT)withV _(DSAT) =V _(G) −V _(T) =V _(G) −V _(T)(V _(DSAT))andV _(T)(V _(X))=V _(T0) +ΔV _(T)(V _(X))As is shown in the above equations, the threshold voltage in the channelis equal to the threshold voltage with the source at zero potentialV_(T0) plus a delta threshold voltage ΔV_(T) which is itself a functionof the voltage in the channel.

The leakage current through the channel under the region 68 of trappedcharge, plotted as a function of the voltage V_(TC), across the portionof the channel under the charge trapping region 68 while reading in thereverse direction, is shown in FIG. 9. From the graph, one can see thatthe approximate leakage current I_(L) through the channel when V_(TC) is2V is 10⁻⁵ A. In the case of the prior art memory cell read in theforward direction, the voltage across the portion of the channel underregion 68 of trapped charge is approximately 2V. In contrast, thevoltage V_(X) in the channel of the memory device of the presentinvention at location 27 beneath the edge 25 of the region 68 of trappedcharge is not 2V but something less, 1V for example. The leakage currentI_(L) corresponding to 1V across the trapped charge region isapproximately 10⁻⁷ A, a whole two orders of magnitude smaller.

Of importance, the edge of the region of trapped charge formed in thenitride layer during programming is the portion of the trapped chargethat begins to affect the gate voltage required to invert the channelbeneath that point.

A graph illustrating the gate voltage required to sustain a givenvoltage in the channel, V_(X), spanning the distance from the drain tothe edge 27 of the channel under the edge 25 of the charge trapping areafor one of the two bits while reading in the reverse direction is shownin FIG. 10. The gate voltage V_(G) that is required to sustain aparticular V_(X) at the point 27 in the channel under the edge 25 of thecharge trapping area 68 (FIG. 5B) is a function of the number ofacceptors N_(A) in the substrate and the thickness of the oxide T_(OX)and is represented by the dashed/dotted line. The solid line representsthe threshold voltage in the channel that exists when the back biaseffect on the threshold voltage is zero. In this case, the thresholdvoltage is constant along the entire channel. However, once there is avoltage in the channel, the threshold voltage is not constant along thechannel. As shown in the graph, the threshold voltage increasesnonlinearly as the voltage in the channel increases. The relationshipbetween the incremental increase in threshold voltage as a function ofchannel voltage is well known in the art. A more detailed discussion ofthis relationship can be found in Chapter 2 of “The Design and Analysisof VLSI Circuits” by L. A. Glasser and D. W. Dobberpuhl, incorporatedherein by reference.

It is important to emphasize that the advantages and benefits of readingin the reverse direction are achieved only when combined with the use ofrelatively low gate voltages. For a particular drain voltage, e.g., 2V,applying a high enough V_(G) such as 5V, for example, causes thedifferences in threshold voltages between forward and reverse reading tofade. A graph illustrating the effect of the gate voltage V_(G) appliedduring reading on the difference in drain current I_(D) between readingin the forward direction versus reading in the reverse direction for oneof the two bits is shown in FIG. 11. The reverse V_(T) of the deviceused to generate the curves in the Figure is 3.5V. From FIG. 11 it canbe seen that as V_(G) is increased while V_(D) is kept constant, theI_(D) curves for the reverse read begin to resemble the curves for theforward read. For example, comparing the forward and reverse read curveswhen V_(G) equals 2.5V shows the read current in the reverse directionbeing about four orders of magnitude lower. At a gate voltage V_(G) of3V, the difference in read current between the forward and reversedirections drops to a little more than two orders of magnitude. At agate voltage of 5V, the difference in read current is only approximately15%. These curves clearly show that large differences in I_(D) betweenthe forward and reverse read directions are only obtained when V_(G) ischosen to be low enough. Thus, the benefits of reading in the reversedirection are only achieved when suitably low gate voltages are used forreading. There is an optimum range within which V_(G) should lie. IfV_(G) is too low, insufficient current is developed in the channel. Onthe other hand, if V_(G) is chosen too high, the differences betweenreading in the reverse and forward directions are greatly diminished.

A graph illustrating the effect of the gate voltage on the difference inthreshold voltage between the forward and reverse directions is shown inFIG. 12. The device used to generate the curves in FIG. 12 wasprogrammed once to a V_(T) of 3.5V using a V_(D) of 1.6V and an I_(TH)of 1 μA. The V_(T) as a function of V_(D) during reading wassubsequently measured. As labeled in FIG. 12, the I_(TH) level for thelower two curves is 1 μA, and is 40 μA for the upper two curves. Theeffect of raising the I_(TH) is to force the V_(T), measurement to be ata higher V_(G) level even though the amount of charge trapped in thesilicon nitride layer is identical for all measurements. For the lowertwo curves (I_(TH) of 1 μA) the forward and reverse threshold voltagesstart to separate from each other at a V_(D) of approximately 50 mVwhile the V_(T), for the reverse saturates at approximately 0.6 V. Forthe upper two curves (I_(TH) of 40 μA) the forward and reverse thresholdvoltages start to separate from each other at a V_(D) of approximately50 mV while the V_(T), for the reverse saturates at approximately 0.6V.For the upper two curves (I_(TH) of 40 μA) the forward and reversethreshold voltages start to separate from each other at a V_(D) ofapproximately 0.35V while the V_(T) for the reverse saturates atapproximately 1.35V. Thus, these curves clearly show that the effect ofthe trapped charge depends heavily on the choice of V_(G).

Programming the Two Bit Cell

With reference to FIG. 2, programming the two bit EEPROM cell of thepresent invention will now be described. In programming the two bitcell, each bit, i.e., the left and right bit, is treated as if thedevice was a single bit device. In other words, both the left and rightbits are programmed as described in the section entitled “ProgrammingOne Bit in the Forward Direction.” For the right bit, for example,programming voltages are applied to the gate 24 and drain 16 and hotelectrons are injected into and trapped in the charge trapping layer 20in the region near the drain defined by the dashed circle 23.Correspondingly, the threshold voltage of the portion of the channelunder the trapped charge increases as more and more electrons areinjected into the nitride layer. The programming of the right bit isrepresented in FIG. 2 by the right-pointing arrow labeled ‘PROGRAM.’This arrow represents the flow of electrons to the right duringprogramming of the right bit.

Similarly, the left bit is programmed by applying programming voltagesto the gate 24 and source 14, which now functions as the drain for theleft bit. Hot electrons are injected into and trapped in the chargetrapping layer 20 in the region defined by the dashed circle 21. Thethreshold voltage of the portion of the channel under the trapped chargecomprising the left bit increases as more and more electrons areinjected into the nitride layer. The programming of the left bit isrepresented in FIG. 2 by the left-pointing arrow labeled ‘PROGRAM.’ Thisarrow represents the flow of electrons to the left during programming ofthe left bit.

A graph illustrating the effect programming one of the bits has on theother bit which has not been previously programmed is shown in FIG. 13.In this particular example, the right bit is shown being programmedwhile the left bit is read. The threshold voltage V_(T) for the rightbit assumes that the right bit is read in the reverse direction to theprogramming direction. Thus the threshold voltage for a programmed leftbit will be relatively low compared to the threshold voltage for theright bit and thus the state of the right bit can be read withoutinterference from the left bit. It is clear from the curves that duringprogramming of the right bit, the unprogrammed left bit remainsunprogrammed. This graph also illustrates the read through of theprogrammed right bit in order to perform a read of the left bit.

A graph illustrating the effect programming one of the bits has on theother bit which has been previously programmed is shown in FIG. 14. Thisgraph was generated in two passes. Each curve is labeled either PASS #1or PASS #2. During the first pass, the right bit was programmed whilereading the unprogrammed left bit, as shown by the curves labeled RIGHTBIT-PASS #1 and LEFT BIT-PASS #1. These curves are similar to the curvesof FIG. 13. During the second pass, once the right bit is programmed,the left bit, previously unprogrammed, is now programmed. At the sametime, the right bit is read. The second pass is represented by thecurves RIGHT BIT-PASS #2 and LEFT BIT-PASS #2.

As shown in FIG. 14, during the first pass, the left bit remainsunprogrammed during the programming of the right bit. Programming theright bit does not affect the unprogrammed left bit. During the secondpass, the left bit is programmed and the right bit remains programmedand can still be read. The gate voltage during programming issufficiently high (typically around 10V) that the programmed right bitdoes not interfere with the programming of the left bit except toincrease somewhat the time required to reach a given threshold voltagerelative to the time required to reach the same threshold voltage forthe right bit when the right bit is programmed. The graph also showsthat the right bit can be programmed during programming of the left bit.Further, the programming of the left bit does not disturb the programmedright bit. This is possible because program through (i.e. theprogramming of the one bit substantially without interference from theother bit when the other bit is programmed) and read through (i.e. thereading of one bit without interference from the other bit when theother bit is programmed) occurs through both the left and the rightbits.

Program through and read through are possible due to the relatively lowgate voltages required to turn on each programmed bit when read in theforward direction as occurs when the other bit is read in the reversedirection. Another way to look at this is that a narrow charge trappingregion permits punch through to be more effective. Thus the small amountof charge 68 trapped on the right edge of charge trapping layer 20 (FIG.15) and self-aligned with the junction between region 16 and thesubstrate 12 and a comparable amount of charge 70 trapped on the leftedge of charge trapping layer 20 and self-aligned with the junctionbetween region 14 and the substrate 12 cause a narrow charge trappingregion to be formed at both the right side and the left side of chargetrapping layer 20 which is easy to be punched through when the bit isread in the forward direction. Thus when left bit 70 (the chargetrapping region 70 is referred to as a bit because the presence orabsence of charge in region 70 would represent either a zero or a one)is read in the forward direction, bit 68 is being read in the reversedirection. The punch-through under charge trap region 70 is quite easilyachieved with a low gate voltage thereby allowing the charge trapped inbit 68 to control the state of the signal read out of the device. Thusfor equal amounts of charge trapped in regions 70 and 68, reading a bitin the reverse direction results in the opposite bit having no effect onthe state of the signal being read.

Another reason that the bit not being programmed is not disturbed isthat the programming voltage is not being applied to the drain for thebit previously programmed. When programming the other bit, theprogramming voltage is applied to the drain for the bit on the otherside of the device.

As discussed earlier, the programming duration must be limited for eachbit in order that the other bit can still be read. For example, in thecase when the right bit is programmed, i.e., a logic ‘0’, and the leftbit is not programmed, i.e., a logic ‘1’, if the right bit wasprogrammed for too long a time then when the left bit is read, there maybe insufficient current for the sense amps to detect a logic ‘1’ becausethe channel is not sufficiently conductive. In other words, if the rightbit is programmed too long, a left logic ‘1’ bit becomes slower, i.e.,takes longer to read due to lower channel current, or, in the worstcase, may appear to be a logic ‘0’ because the over-programmed right bitprevents the left bit from being read. Thus, a window exists in theprogramming time within which a logic ‘0’ bit must fall. One of thevariable parameters is the voltage that is applied to the functionaldrain region during read. As the drain voltage is increased, a longerprogramming time, i.e., longer area of trapped charge, is required inorder to avoid punch through. Thus, a longer trapping region isequivalent to increasing the programming time. The upper limit of theprogramming time for the window is the programming time such that aforward read does not change the read current by more than apredetermined percentage compared to the read current for a reverseread. Preferably, the percentage change to the read current should belimited to 10%. This percentage, although not arbitrary, can beoptimized according to the design goals of the chip designer. Forexample, a designer may wish to have three orders of magnitude marginbetween the threshold voltage of a forward read and the threshold for areverse read. To achieve this, the gate voltage, drain voltage andimplant level are all adjusted accordingly to determine a maximumprogramming time.

The effect of programming one of the bits is that both programming andreading for the second bit is slowed somewhat. The second bit can beprogrammed as long as the gate voltage during programming is higher thanthe threshold voltage of the channel with the first bit programmed andsufficient voltage is placed on the drain. The channel resistance,however, is raised due to the programming of the fist bit. As long asprogramming parameters are tuned properly, the higher channel resistancedoes not prevent the second bit from being programmed and read. Thehigher channel resistance, however, does cause programming and readingof the second bit to take longer.

Reading the Two Bit Memory Cell

Reading the two bit EEPROM cell of the present invention will now bedescribed. In reading the two bit cell, as in programming, each bit istreated as if the device was a single bit device. A sectional view of atwo bit EEPROM cell constructed in accordance with a preferredembodiment of the present invention showing the area of charge trappingunder the gate for both the right and the left bits is shown in FIG. 15.The area of trapping for the right bit is referenced 68 and that of theleft bit is referenced 70. Also shown in FIG. 15 are two arrows labeled‘READ’, one pointed in the left direction indicating the direction forreading of the right bit and one pointed in the right directionindicating the direction for reading of the left bit.

As described in the section entitled “Reading One Bit in the ReverseDirection” the right bit is read in the reverse direction by applyingread voltages to the source 14 and the gate 24 and grounding the drain16. For example, a gate voltage of 3V and a source voltage of 2V isapplied. The resulting voltage in the channel V_(X) will be somethingless than two volts in accordance with the graph in FIG. 10 and asdescribed in detail above. Similarly, to read the left bit in thereverse direction, read voltages are applied to the gate 24 and to thedrain 16 and the source 14 is grounded, e.g., 3V on the gate and 2V onthe drain.

A graph illustrating the effect of a low drain voltage on the readthrough of a programmed bit is shown in FIG. 16. This graph is similarto that of FIG. 14 with the addition of the top two curves above 5.1V.The four lower curves were generated using a V_(D) of 1.6V. The twoupper curves were generated by reading the unprogrammed bit after theother bit was programmed using a V_(D) of 50 mV. These curves show thatif V_(D) is made too low and the first bit is programmed, insufficientvoltage exists in the channel for read through to occur. They also showthat the second bit to be programmed, in this case the left bit,experiences slower programming due to the increased series resistance ofthe channel. Even if the second bit is unprogrammed, when the drainvoltage is too low and the first bit is programmed, the second bitcannot be read properly. Insufficient voltage exists in order for punchthrough to occur. If punch though does not occur, the second bit looksas if it is programmed whether it really is or not.

Punch through is very sensitive to the length of the trapped chargeregion, such as regions 68 and 70 of the structure shown in FIG. 15.Should these regions be too wide or not self-aligned with theappropriate region 16 or 14 (depending on whether the charge representsthe right bit 68 or the left bit 70), then punch through would not beable to be guaranteed to occur and this concept would not work. Thus,the self-alignment of the trapped charge to the junction between region16 and the substrate (for the trapped charge 68) and region 14 and thesubstrate (for the trapped charge region 70) is crucial to thefunctioning of this invention.

A read of the two bit memory device of the present invention falls intoone of three cases: (1) neither of the two bits are programmed (2) oneof the bits is programmed and the other is not or (3) both of the bitsare programmed. The first case does not require a read through. Thesecond case requires reading through the programmed bit to read theunprogrammed bit. In this case the margin is the delta between reading asingle bit in the forward direction versus the reverse direction. Anexample of the margin can be seen in FIGS. 6 and 7 which illustrate thedifference in V_(T) and read current between the forward and the reversedirections for a single bit.

The third case requires read through to read both programmed bits.Programming the second bit, in fact, improves the conditions for readingthe first bit. This is so because the voltage in the channel is furtherreduced over the case of reading a single bit. This increases the readmargins between programmed and unprogrammed bits.

It is important to note that although the EEPROM cell of the presentinvention stores two bits, support circuitry and concepts designed towork with single bit memory cells can still be used. For example, thesense amplifier circuitry needed for the two bit memory cell isbasically no different than that for the single bit memory cell. In thesingle bit memory cell, the sense amplifier circuitry is required todistinguish between two states, the programmed and unprogrammed states.Likewise, in the two bit memory cell of the present invention, the senseamplifiers must also distinguish between only two states: programmed andunprogrammed. This is in direct contrast to the prior art approaches tomulti-bit memory cells wherein multiple thresholds are used whichrequire multiple current levels to be detected by the sense amps.Accurately detecting multiple current levels in a memory device is acomplex and difficult task to accomplish. The memory cell of the presentinvention, however, requires that the sense amps only distinguishbetween two states as in the single bit memory cell.

In the case when one of the bits is unprogrammed, i.e., no chargeinjected into charge trapping layer for that bit, a read of the otherbit will be unaffected by this unprogrammed bit. On the other hand,however, in the case when one bit is programmed, a read of the other bitwill be affected by this other programmed bit to some extent. Dependingon various process parameters, the programmed bit may cause the channelto be less conductive. However, as long as the channel is sufficientlyconductive both bits can be still be programmed and read correctly. Thisis discussed in more detail in the section titled “OptimizationParameters” presented below.

With reference to FIG. 15, the two bit memory device of the presentinvention utilizes a punch through or read through technique to read onebit when the other bit is in a programmed state. In order to read, forexample, the right bit 68, the read current must be able to read throughor punch through the left bit 70, assuming that both the left bit andthe right bit have been programmed. Thus, there is a limit on the lengthof the charge trapping region that can be programmed. The chargetrapping region must be short enough to permit punch through of the bitnot being read. If a bit is in the unprogrammed state, there is noconstraint on the read current of the other bit from the unprogrammedbit.

It is important to note that when a semiconductor device is scaled, thechannel lengths become shorter and short channel effects take hold.Thus, in the two bit memory cell, because each bit is stored indifferent areas of the transistor, short channel effects may becomeprevalent sooner than in the case of the single bit transistor. In orderto retain the usable range of drain voltage, the two bit transistor mayneed to be scaled by a smaller factor.

Criteria Necessary for Two Bit Operation

A key concept associated with the two bit EEPROM memory cell of thepresent invention is that for the device to operate properly, both bitsmust be able to be written and read. If one of the bits is programmed, areverse read on the programmed bit must sense a high V_(T), i.e., a ‘0’and a reverse read on the unprogrammed bit must sense a low V_(T), i.e.,a ‘1’. Thus, a reverse read on the unprogrammed bit, which is equivalentto a forward read on the programmed bit, must punch through the regionof trapped charge in order to generate a high enough read current. Ifthis does not happen, the unprogrammed bit will not be able to be readas a ‘1’, i.e., a conductive bit.

In order to achieve this goal, a sufficient margin is generated betweenreading in the forward and reverse directions. With reference to FIG.11, in order to store two bits, there must be sufficient differencebetween forward read of one of the bits and reverse read of the otherbit. In addition, the reverse read current for one of the bits when theother bit is and is not programmed should be sufficient to distinguishbetween the two bits. For example, in FIG. 11, for a gate voltage of 3V,punch through for reading in the reverse direction occurs atapproximately 1V. Thus, a drain voltage of 1.6V creates a suitablesafety margin ensuring that the second bit can be read when the firstbit is programmed.

There are two parameters that can be used to ensure punch through of thecharge trapping region. The first is the V_(G), applied during readingand the second is the width of the charge trapping region. A low V_(G)used during reading combined with a narrow charge trapping region makespunch through more effective. The lower gate voltage produces a weakervertical electric field which causes the lateral electric field to bestronger.

It is more important to use a low V_(G) during reading in the two bitmemory cell than in the single bit memory cell. In the single bit case,it only had to be ensured that the reverse read was better than theforward read, meaning that the V_(T) of a given bit during forwardreading was lower than the V_(T) of this bit during reverse reading. Inthe two bit case, however, it is not enough that the V_(T) drops in theforward case, it must drop sufficiently to be able to punch through whenreading the other bit. If the delta V_(T) between the forward andreverse read is not sufficient, one bit cannot be read when the otherbit is programmed.

Erasing Prior Art Memory Devices

As discussed previously in connection with U.S. Pat. No. 4,173,766,issued to Hayes, a major disadvantage of the Hayes prior art insulatedgate device is the difficulty in using the Hayes device to construct aflash EEPROM. A consequence of using an oxide-nitride structure asopposed to an oxide-nitride-oxide structure is that during programmingthe charge gets distributed across the entire nitride layer. The absenceof the top oxide layer lowers the ability to control where the charge isstored in the nitride layer and allows holes from the gate to neutralizecharge in the nitride layer. A thick nitride layer is required in orderto generate sufficient charge retention in the device. However, therelatively thick nitride layer causes the charge trapping region to bevery wide thus making erasing the cell difficult if not impossible. Thusthere is a tradeoff between charge retention and sufficiently largethreshold voltage deltas on the one hand and the ability to erase thedevice on the other hand.

Some of the prior art devices that use hot electron programming utilizean erase mechanism whereby the electrons previously trapped in thenitride are neutralized (i.e., erased) by transferring holes into thenitride. The information is erased by grounding the gate and applying asufficient potential to the drain to cause avalanche breakdown.Avalanche breakdown involves hot hole injection and requires relativelyhigh voltages on the drain for the phenomenon to occur. The hot holesare generated and caused to jump over the hole potential barrier of thebottom oxide between the channel and the nitride and recombine with theelectrons in the nitride. This mechanism, however, is very complex andit is difficult to construct memory devices that work in this manner.Another disadvantage of using hot hole injection for erasing is thatsince the drain/substrate junction is in breakdown, very large currentsare generated that are difficult to control. Further, the number ofprogram/erase cycles that the memory cell can sustain is limited becausethe breakdown damages the junction area. The damage is caused by veryhigh local temperatures generated in the vicinity of the junction whenit is in breakdown.

Erasing the Two Bit Memory Cell

The erase mechanism of the two bit flash EEPROM memory cell 10 (FIG. 15)will now be described in more detail. The mechanism used to erase thetwo bit flash EEPROM memory cell of the present invention involves themovement of electrons as opposed to the movement of holes. For the rightbit, an erase is performed by removing electrons from the chargetrapping nitride region 68 either through the gate 24 via the top oxide22 or through the drain 16 via the bottom oxide 18. For the left bit, anerase is performed by removing electrons from the charge trappingnitride region 70 either through the gate 24 via the top oxide 22 orthrough the source 14 via the bottom oxide 18.

Using the right bit as an example, one technique of erasing is tosimultaneously apply a negative potential to the gate 24 and a positivepotential to the drain 16 such that electron tunneling occurs from thecharge trapping nitride layer 20 to the drain 16 via the bottom oxide18. The left bit is erased in a similar fashion except that a positivepotential is applied to the source 14 rather than the drain 16. Theelectron tunneling is substantially confined to a local area near thedrain 16. To facilitate the erasing of the memory cell 10 using thistechnique, the thickness of the bottom oxide layer 18 is suitablyconstructed (i.e., has a thickness of about seventy (70) Angstroms) tooptimize the removal of electrons from the nitride charge trapping layer20 into the drain 16.

Using the right bit as an example, a second well known technique is tosimultaneously apply a positive voltage potential to the gate 24 andzero potential, i.e., ground, to the drain 16 such that electrontunneling occurs from the charge trapping nitride layer 20 through thetop oxide 22 to the gate 24. The right bit is erased in a similarfashion with zero potential applied to the source 14. In this case, thetop oxide 22 is suitably constructed (again with a thickness of aboutseventy (70) Angstroms) to optimize the tunneling of electrons from thenitride charge trapping layer 20 into the gate 24 in order to facilitatethe erasing of the memory cell 10. In one embodiment, the top oxide 22has a thickness of 50 Angstroms to 80 Angstroms for a voltage on gate 24of 10 to 18 volts.

A graph illustrating the effect of programming on erase time for readingin the forward and reverse directions is shown in FIG. 17. FIG. 17 showsthe times necessary to program the device to a threshold voltage of four(4) volts for reading in both the reverse (10⁻⁵ seconds) and forward(3×10⁻³ seconds) directions. The graph presented in FIG. 17 is based ondata obtained from a memory cell constructed in accordance with thepresent invention. In the first pass, the device was programmed to beread in the reverse direction and then erased. In the second pass, thedevice was programmed to be read in the forward direction and thenerased. The erase processes for the charges associated with reverse readand forward read used the same drain voltage and gate voltage, namely aV_(D) of 5.5V and a V_(G) of −8V. The thickness of the top oxide, bottomoxide and nitride layers are all 100 Angstroms. Programming for forwardreading and reverse reading utilized a V_(D) of 5.5V and V_(G) of 10V.Only the programming times differed. The forward and reverse programmingcurves are identical to those illustrated in the graph of FIG. 8.

As can be seen from FIG. 17, even when the device is programmed to thesame threshold voltage, the time to complete the reverse erase is muchless than the time to complete the forward erase. The forward erase(i.e. the time to remove the trapped charge associated with a giventhreshold voltage when the device is read in the forward direction) isslower than the reverse erase (i.e. the time to remove the trappedcharge associated with a given threshold voltage when the device is readin the reverse direction). In addition, there is residual charge left inthe charge trapping region as shown in the small gap between the reverseand forward erase curves at the one (1) second mark. This is due to thelarger wider charge trapping region formed during the forwardprogramming that was required to generate a threshold voltage of 4V.From the curves, the forward erase is approximately an order ofmagnitude slower than the reverse erase. The abrupt increase inthreshold voltage for the curve labeled ‘FORWARD ERASE’ is due to thereverse read used to measure the threshold voltage. For the same amountof charge trapping, the equivalent threshold voltage for reverse readingis much higher than that for forward reading. As can be seen in FIG. 17,the slopes of the forward and reverse erase curves are different.Reading in the reverse direction requires trapped charge so much smallerthan does reading in the forward direction that the erase of the trappedcharge is approximately 10 to 20 times faster. Also apparent from FIG.17 is that the cell does not enter deep depletion. Even at the 1 seconderase mark, the threshold voltage (about 2v) is no lower than that of anunprogrammed cell. This is a huge advantage of the memory cell of thepresent invention over prior art memory cells especially floating gatecells where over-erase can cause a failure of the memory array due todeep depletion of the charge on the floating gate.

The erase mechanism in the memory cell is self limiting due to the factthat as the memory cell is erased, more and more positive charge isstored in the trapping region 68 (FIG. 15) (for the right bit) of thenitride layer thereby neutralizing the negative charge stored therewhile the remainder of the nitride layer 20 remains unaffected. Thus,the threshold voltage of the channel keeps dropping until it levels offat the threshold voltage of an unprogrammed memory cell which is thethreshold voltage of the larger majority of the channel closer to thesource. Over-erasing the memory cell of the present invention onlyaffects (i.e., lowers) the threshold voltage of the portion of thechannel under the charge trapping region 68 which is a relatively narrowregion while leaving the threshold voltage of the remainder of thechannel at its normal value. A graph illustrating the separate bit erasecapability of the two bit EEPROM memory cell of the present invention isshown in FIG. 18. The graph was generated in two passes and initially,both the right and the left bit are programmed each with an amount oftrapped charge to achieve a given threshold voltage when read in thereverse direction. During the first pass, the right bit was erased whilethe left bit was read, as represented by the curves labeled RIGHTBIT-PASS #1 and LEFT BIT-PASS #1. During the second pass, the left bitwas erased while the right bit was read, as represented by the curveslabeled RIGHT BIT-PASS #2 and LEFT BIT-PASS #2. The graph shows thaterasing of one of the bits does not affect the other bit. This is due tothe fact that the erase voltage is localized to the junction adjacent tothe bit that is to be erased. The difference in location between thecurve labeled “Left Bit-Pass #2” and the curve labeled “Right Bit-Pass#1” is of no significance being well within the tolerance of themeasurements.

A graph illustrating the effect of cycling on the program and eraseability of the two bit EEPROM cell of the present invention is shown inFIG. 19. The graph shows the V_(T) of a bit associated with a givenamount of trapped charge for reading in the reverse direction (top line)and the forward direction (bottom line) The gradual increase inthreshold voltage V_(T) for reading in both the forward and reversedirections reflects the lack of complete erasure of all the storedcharge during each erase such that the amount of trapped chargegradually increases with time after programming and erasing for 1000cycles.

As explained previously, a result of reading in the reverse direction isthat a narrower charge trapping region is required due to the higherefficiency of the reverse read. Since erasing is always performedthrough the effective drain region 16 (for trapped charge 68 and region14 for trapped charge 70), less charge needs to be moved off the chargetrapping layer 20 and directed through the drain 16 (charge 68) oreffective drain 14 (charge 70). Thus, reading the memory cell 10 in thereverse direction enables much faster erase times. This makes the entireerase process much easier than in prior art memory devices. In the priorart memory device (i.e., forward programming/forward read), the chargetrapping region 66 (FIG. 5A) was much bigger and wider to achieve thedesired change in threshold voltage, thus making the erase process moredifficult. To erase the cell 41, a larger amount of charge spread outover a wider trapping region 66 must be directed through the drain 34.The danger with this lies in that if the charge trapping region 66becomes too wide, the cell 41 may never be able to be completely erased.The charge trapping region 66 may become too wide if the device isoverprogrammed which is a real possibility when programming and readingin the forward direction.

A graph illustrating the effects associated with over programming on theability to erase in the forward and reverse directions is shown in FIG.20. The graph presented in FIG. 20 was constructed using data obtainedfrom a memory cell 10 (FIGS. 5B and 15) constructed in accordance withthe present invention. The top oxide 22 (FIG. 15), bottom oxide 18 andnitride layer 20 are each 100 Angstroms thick for a total ONO thicknessof 300 Angstroms. Programming utilized a V_(D) of 5.0V and V_(G) Of 10V.Erasing utilized a V_(D) of 5.0V and a V_(G) of −8V. Note thatprogramming and erasing are both in the forward direction. Reading,however, is either in the forward or reverse direction. It is thereverse direction read in conjunction with the careful control of thegate voltage to be within a selected range, that yields the advantagesof this invention.

In this case, the memory cell, which has been programmed for 100milliseconds, does not fully erase in a reasonable time (shown in FIG.20 as 100 milliseconds) with V_(T) being approximately 7V after 100milliseconds of erase for reading in both the forward and reversedirections. The cell 10 cannot be erased because it has been overprogrammed, meaning the charge trapping region was made too wide toeffectively erase. After 100 milliseconds of programming, the chargetrapping region is very wide. The 13V (V_(D) of 5V and V_(G) of −8 v)that is applied across the charge trapping region 68 (FIG. 5B) to erasethe trapped charge is effective in removing the electrons that are closeto the drain 16. However, the electrons that are trapped further awayfrom drain 16 towards the middle of the channel cannot be effectivelyremoved because the electric field created by the 13V potentialdifference between the drain and the gate is much weaker at that point.

As is apparent from FIGS. 17 and 20, the slopes of the threshold voltageV_(T) versus program time curves for forward read and reverse read(labeled “forward program” and “reverse program” in FIG. 20) aredifferent. After approximately one millisecond, the forward programcurve exhibits a higher slope than the reverse program curve. This showsthat reading in the reverse direction is more tolerant to overprogramming than reading in the forward direction in the sense that agiven uncertainty in programming time causes a bigger uncertainty inthreshold voltage V_(T) when reading in the forward direction than whenreading in the reverse direction. When reading in the reverse direction,a V_(T), of about 4V is reached after approximately 100 microseconds ofprogramming. Even if programming continues up until a millisecond, afactor of 10×, the V_(T), for reading in the reverse direction is onlyapproximately 4.5V. For reading in the forward direction, a V_(T) of 4Vis reached only after approximately 7 milliseconds of programming. Ifprogramming is off by only 3×, the V_(T) increases to approximately8.3V. At this high V_(T) it is not likely that the device can be erased.

Thus, it is important to stress that reading the memory device in thereverse direction does not just enable simpler and faster erasing, butin fact, if the device is to be read in the forward direction and thetrapped charge is so adjusted to give the desired threshold voltageV_(T), erasing is likely to be not possible at all. This is because muchmore charge must be trapped on the dielectric 20 beneath the gate 24 toachieve a usable difference in threshold voltage V_(T) between theprogrammed and the unprogrammed state when reading in the forwarddirection than when reading in the reverse direction. This makes erasingthe memory device at best difficult if not impossible thus making theforward programming/forward read impractical for this type of memorydevice which must be erasable.

The graph of FIG. 20 also illustrates the higher effectiveness duringerase of the voltage on the drain versus the voltage on the gate. Thegate voltage is not as effective due to the distance of the gate fromthe trapped charge which includes the thickness of the top oxide 22 andthe nitride layer 20. The drain voltage is more effective since it ismore proximate to the region 68 of trapped charge. However, the gatevoltage is more crucial when the width of the trapped charge region 68is narrow. In this case, the gate voltage will be effective in creatingan electric field that covers the entire charge trapping region 68making the removal of electrons more efficient. The trapped chargeregion can only be made sufficiently narrow if the device is read in thereverse direction because only when the device is read in the reversedirection does a relatively small amount of charge stored on thedielectric under the gate yield a sufficiently larger difference inthreshold voltage VT to allow the programmed state (i.e., charge storedon the gate) and the unprogrammed state (i.e., no charge stored on thegate) to be differentiated. As discussed previously, if the device isread in the forward direction, the charge trapping region must be madewide enough to generate a sufficient threshold voltage to differentiatebetween the programming and the unprogrammed states. Charge trapped farfrom the drain cannot be compensated for by lowering the voltage on thegate. In addition, the drain voltage cannot be increased beyondapproximately 2V due to read disturb. The read disturb refers to slowprogramming of the bit during read. While the programming occurs veryslowing, constantly reading the same cell over an extended period oftime can cause programming of the bit to occur.

A graph illustrating the programming and erasing curves representing theuse of oxide versus TEOS as the dielectric on top of the nitride isshown in FIG. 21. The graph presented in FIG. 21 was constructed usingdata obtained from two memory cells constructed in accordance with thepresent invention, one memory cell using TEOS to form the oxide layer 22(FIG. 15) on top of the nitride and the other memory cell using thermaloxidation of the nitride to form the top oxide layer 22. The thickness'of the top oxide layer 22, bottom oxide layer 18 and nitride layer 20are 70, 100, 80 Angstroms, respectively. The width/length ratio for eachmemory cell channel is 0.6/0.65 microns. Programming (which is alwaysdone in the forward direction) utilized a V_(D), of 5.0V and a V_(G) of10V. Erasing (which is also always done in the forward direction)utilized a V_(D) of 5.0V and a V_(G) of −6V. This graph shows that thereis little difference in the programming and erase characteristics wheneither oxide or TEOS is placed on top of the nitride.

A graph illustrating erase times for a gate voltage of zero with twodifferent values of drain voltage is shown in FIG. 22. Both curves weregenerated by first programming in the forward direction for about 10microseconds, until the threshold voltage V_(T) equals about 4V and thenerasing in the forward direction. For the upper curve, the gate 24 (FIG.15) was grounded and 6.0V applied to the drain 16. For the lower curve,the gate 24 was grounded and 6.5V applied to the drain. For both curves,the threshold voltage is raised during programming from nearly 1.5V toapproximately 4V. Erasing then brings the V_(T) back down toapproximately 1.7V. Note that the time to erase the charge from thedielectric decreases as the drain voltage increases. The curves showthat it takes about 100 seconds with a gate voltage of 6.5V to erase(i.e., remove) sufficient charge from the dielectric to bring thethreshold voltage of the device down to about 1.9V and that it takesabout 1000 seconds with a gate voltage of 6.0V to achieve the samethreshold voltage.

A graph illustrating the erase curve for two different values ofnegative gate voltage is shown in FIG. 23. The graph presented in FIG.23 was constructed using data obtained from a memory cell constructed inaccordance with the present invention. The thickness of each of the topoxide 22 (FIG. 15), bottom oxide 18 and nitride 20 layers is 100Angstroms for a total dielectric thickness of 300 Angstroms. The channelwidth/length ratio is 0.6/0.65 microns. Erasing for the reversedirection utilized a constant V_(D) of 5.5V and a V_(G) of −5V versus aV_(G) of −7.5V. The graph shows that drain and gate voltages on theorder of 5V and −5V respectively, are sufficient to enable an effectiveerase. This is a big advantage over the prior art where erase voltagesof around −10V on the gate are more typical. The graph also shows thatlowering V_(G) to −7.5V is effective to erase the device approximately20 times faster while still retaining a V_(G) less than 10V.

Benefits of Reading in the Reverse Direction

Reading the graph in FIG. 10, one can see that to achieve a V_(X) equalto approximately 2V in the channel (i.e., the same conditions as theprior art memory device with 3V applied to the gate) when reading in thereverse direction, approximately 4V must be applied to the gate. When,for example, 3V is applied to the gate and the device is read in thereverse direction, only approximately 1.2V is generated in the channel.This is in direct contrast to the prior art reading in the forwarddirection wherein the potential across the trapped charge region wasalmost the full potential applied to the drain (i.e., 2V). Thissignificant benefit of reading in the reverse direction is that for thesame gate voltage a much lower voltage is present across the portion ofthe channel under the region of trapped charge. This results indramatically less leakage current for the same charge trapping length.Or stated another way, a shorter charge trapping region is needed in thegate dielectric to achieve an equivalent amount of leakage current. Ashorter charge trapping region translates through an exponentialfunction to shorter programming times. A discussion of the variation inprogramming time as a function of various parameters, voltage andtemperature is given in a paper entitled “Hot-Electron Injection Intothe Oxide in n-Channel MOS Devices,” B. Eitan and D.Frohman-Bentchkowsky, IEEE Transactions on Electron Devices, March 1981,incorporated herein by reference.

The effect of reading the memory device in the reverse direction is toamplify the effect of the trapped charge (i.e., the programmed region orthe localized trapping region) on the threshold voltage thereby allowingmuch less charge to be trapped to achieve the same difference inthreshold voltage between the programmed state (i.e., charge stored inthe charge trapping region of the gate dielectric) and the unprogrammedstate (i.e., no charge stored in the charge trapping region of the gatedielectric) of the device. For the same programming time (meaning thesame length of trapped charge in the nitride, for example as shown inFIGS. 5A and 5B), device 10, when read in the reverse direction,exhibits a leakage current IL approximately two orders of magnitude lessthan that of a prior art memory cell. As previously discussed, byreading in the reverse direction, a major benefit is that theprogramming time can be reduced because the leakage current issignificantly less and thus less trapped charge is required to achievethe same leakage current as when reading in the forward direction. Thus,the size of the trapping region does not have to be as large as withprior art memory cells which translates exponentially into shorterprogramming times.

A key advantage of reading in the opposite direction from programming isthat the effect of the lateral electric field next to the chargetrapping region is minimized. In addition, the gate voltage can bereduced to further minimize the potential in the channel. In fact, thegate voltage can be set to achieve the desired voltage in the channel.This was described previously with reference to FIG. 10.

The area of charge trapping necessary to program memory cell 41 of theprior art is illustrated in FIG. 24A and the area of charge trappingnecessary to program memory cell 10 of the present invention isillustrated in FIG. 24B. The trapping region 68 of device 10 is shownmuch smaller than trapping region 66 of the prior art device. Asdescribed earlier, reading in the reverse direction permits a shortercharge trapping region. This results in much more efficient programmingby reducing, through an exponential function, the programming time ofthe device.

Programming a smaller, narrower region of trapped charge has numerousbenefits. One major benefit is that programming times are reduced whilethe delta in threshold voltage between the programmed versusunprogrammed states remains the same. Thus, short programming times areachieved by taking advantage of the asymmetric characteristics of thetrapping dielectric flash EEPROM memory cell. Another major benefit isthat the erase mechanism of the memory cell is greatly enhanced.

The erase mechanism is enhanced when the charge trapping region is madeas narrow as possible. Programming in the forward direction and readingin the reverse direction enables limiting the width of the chargetrapping region to a narrow region near the drain. This allows for muchmore rapid and thus more efficient erasing of the memory cell.

Yet another benefit of reading in the reverse direction, as describedabove, is that a narrow charge trapping region increases theeffectiveness of the drain voltage during erase when combined withrelatively low applied gate voltages. A narrow charge trapping region isallowed only by reading in the reverse direction while applying low gatevoltages during the read.

Further, utilizing a thinner silicon nitride charge trapping layer thandisclosed in the prior art helps to confine the charge trapping regionto a region near the drain that is laterally narrower than in the priorart. This improves the retention characteristic of the memory cell.Further, the thinner top and bottom oxide sandwiching the nitride layerhelps retain the vertical electric field.

In addition, when the memory cell is read in the reverse direction, itis more tolerant of over programming. Reading in the forward directioncauses the threshold voltage of the memory cell to be very sensitive toinaccuracies in programming time while reading in the reverse directionreduces this sensitivity of threshold voltage to programming time. Overprogramming while programming to allow reading in the forward directioncan potentially cause the device to become non erasable.

The voltage V_(X) in the channel is a function of the gate voltage andthe impurity level in the channel. V_(X) is the voltage in the channeljust beneath the edge of the trapped charge region above the channel(FIG. 5B). A higher gate voltage translates to a higher voltage in thechannel. When the device is N channel, the impurity in the channelregion before inversion is usually boron. The voltage V_(X) is generallyindependent of the boron impurity level over a normal range of values inthe forward reading mode, but V_(X) is dependent on the impurity levelin the reverse direction, becoming smaller as the impurity level goesup. Indeed in the reverse direction the voltage V_(X) in the channeljust beneath the edge of the trapped charge region is given by thefollowing expressionV _(X) =V _(G)−(V _(T) +ΔV _(T))where V_(T) is the device threshold voltage for zero substrate bias andΔV_(T) is the incremental increase in threshold voltage due to substrateback bias caused by a finite value for V_(X) when the channel is justinverted.

Various thicknesses were tried for the second oxide layer 22 in the ONOstructure of FIGS. 5B and 24B. The following table presents thecombinations of thickness' for the ONO layers that were constructed forthree embodiments of the memory cell of this invention. Note that allthickness' are in Angstroms in the table below. Embodi- Embodi- Embodi-Layer ment #1 ment #2 ment #3 Top Oxide (‘O’ Layer 22) 150 100 70Nitride (‘N’ Layer 20) 50 50 50 Bottom Oxide (‘O’ Layer 18) 70 70 70Total Thickness 270 220 190

The nitride layer 20 retains the stored charge. By employing the reverseread as opposed to the forward read, the amount of charge required to beretained for a given shift in threshold voltage is reduced by a factortypically of two or more. By making the nitride layer 20 thinner and thetop oxide layer 22 thicker, the amount of charge required to be storedon the nitride layer 20 for a given threshold voltage shift is alsoreduced.

It is also noted that as the thickness of the top oxide layer 22increased, the lateral fields associated with the charge stored on the50 Angstrom thick nitride layer 20 decreased slightly. It is alsoobserved that as the thickness of the bottom oxide layer 18 was madethinner, the erase of the charge stored on the nitride layer 20 becomeseasier. For a 70 Angstrom thick bottom oxide layer 18, the charge storedon the nitride layer 20 is more easily erased than if the bottom oxidelayer 18 is 100 Angstroms thick.

Thus, the conclusion is that the thinner the nitride the better for thepurposes of the present invention. Nitride layers as thin as 20Angstroms are believed possible with this invention. The thinner nitridereduces the lateral field associated with a given charge stored in aportion of the nitride layer and thus reduces the lateral dispersion ofthe stored charge as a result of the internally generated electric fieldassociated with the stored charge.

Optimization Parameters

In terms of optimization, three parameters can be varied to give thequickest programming time and the widest margins. The first parameter isthe channel length. A longer channel length, for a given programmingtime when reading in the reverse direction, increases the distancebetween the drain and the trapped charge (effectively, the source anddrain designations are flipped). This lowers the level of the lateralelectric field even lower.

The second parameter, as described previously, is the gate voltage whichcan be set to minimize the voltage drop in the channel across thechannel region beneath the trapped charge. This further reduces thelateral electric field in the channel beneath the trapped charge. Withinlimits, the voltage in the channel can be ‘dialed in’ by varying thevoltage on the gate. This allows control over the voltage drop in thechannel beneath the region of trapped charge. If the gate voltage ismade too low then reading a logic ‘1’, i.e., the unprogrammed state,becomes problematic. The gate voltage for reading a logic ‘1’ must bestill high enough to generate inversion in order to produce sufficientread current for each sense amplifier. Thus, a lower limit for the gatevoltage is approximately 1V above the threshold voltage. The lower limitfor the gate voltage is determined by the maximum time required to sensethe channel current which represents one state of the memory cell. Forexample, for fast access time, the maximum time would be in the range of10 to 30 nanoseconds while for a mass storage device the maximum accesstime could be as high as 1 microsecond. The actual gate voltage toachieve these maximum times would depend upon the device structure, thedielectric thickness, the bit line capacitance, the doping concentrationin the channel and other parameters associated with the device. An upperlimit on the gate voltage is the voltage at which the voltage in thechannel just beneath the edge of the region of trapped charge is justbelow the voltage potential applied to the source terminal duringreading in the reverse direction. A too high gate voltage will causeinversion in the channel and the benefits of the present invention arelost. Thus, it is not recommended to apply a gate voltage that generatessuch a high voltage in the channel beneath the edge of the chargetrapping region because it defeats the benefits of having a lowerpotential across the portion of the channel beneath this charge trappingregion with the accompanying reduction in leakage current and shortenedprogramming time. In a preferred embodiment of the present invention,the gate voltage used for reading is approximately 3V which representsan optimized tradeoff between programming time and leakage current.

The third optimization method, previously described and which is knownin the art, is to vary the boron doping of the channel region under thegate. An increase in the doping concentration results in a higherthreshold voltage V_(T) and a lower voltage generated in the channel.This is due to the reduction in the width of the depletion regionformed. Thus, a higher doping concentration permits a higher gatevoltage to be applied for the same voltage across the portion of thechannel beneath the charge trapping region.

In addition, an increase in the N_(A) doping concentration for the samelength trapping region will improve the punch through behavior of thedevice. By varying the level of boron implanted in the channel region,the width of the depletion region under the gate can be varied. Anincrease in the doping concentration results in a reduction in the widthof the depletion region for the same applied gate voltage. The reductionin the width of the depletion region occurs because there is now morefixed charge in the substrate. Thus, varying the doping concentrationcan be used to limit the length of the pinchoff region under the gate.In addition, the doping concentration can be used to increase ordecrease the initial threshold voltage of the device.

Optimization parameters specific to programming and reading two bits inthe memory cell of the present invention will now be described. Theoptimizations for programming include utilizing a longer minimumeffective channel length Leer in order to physically separate the twobits better. In addition, the implant level can be reduced in thechannel in order to increase the delta between forward and reverseprogramming. On the other hand, the implant level can be increased inthe channel in order to reduce the impact of the first bit on theprogramming of the second bit. Thus, the implant level in the channel isa compromise between the forward and reverse delta on the one hand andthe programming speed on the other hand.

The optimizations for reading include lowering the gate voltage in orderto enhance the punch through during reading. As described previously,punch through is necessary to program and read the second bit. A lowerimplant level in the channel serves to increase punch through. Also, ahigher drain voltage during read functions to increase punch through.These three optimizations relate to reading in the forward direction,which is equivalent to reading the second bit in the reverse.

In addition, a lower gate voltage reduces the number of electrons thatneed to be injected into the charge trapping region. This improveserasing because it eliminates residual charge remaining trapped aftererasure. Any residual charge that remains in the charge trapping layerafter erasure degrades cycling.

While the invention has been described with respect to a limited numberof embodiments, it will be appreciated that many variations,modifications and other applications of the invention may be made.

1-66. (canceled)
 67. A nitride read only memory (NROM) cell comprising:a charge trapping region comprising at least a nitride layer; a drainregion and a source region; and a first and a second charge storage areawithin said nitride layer, said first charge storage area being close tosaid drain region and said second charge storage area being close tosaid source region; wherein each said charge storage area is adapted tobe programmed to different voltage levels; and wherein said drain regionacts as a source and said source region acts as a drain when programmingsaid second charge storage area.
 68. A nitride read only memory (NROM)cell comprising: a charge trapping region comprising at least a nitridelayer; a drain region and a source region; and a first and a secondcharge storage area within said nitride layer, said first charge storagearea being close to said drain region and said second charge storagearea being close to said source region; wherein each said charge storagearea is adapted to store multi-level bits; and wherein said drain regionacts as a source and said source region acts as a drain when programmingsaid second charge storage area.